Multilayer composite and a method of making such

ABSTRACT

A multilayer composite includes at least two composites, each composite having a film and an electronically conductive layer. Several composites are laminated to provide an increased conversion between mechanical and electrical energies not only due to the multiplication of the effect of each layer, but also due to the fact that the multilayer structure itself renders the multilayer composite more rigid. In addition, the multilayer structure facilitates application of an electrical field over thinner portions of the structure, thereby requiring much less potential difference between electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of currently pending U.S. applicationSer. No. 11/592,651, filed Nov. 3, 2006, which is a continuation-in-partof U.S. application Ser. Nos. 10/415,631, filed Aug. 12, 2003;10/499,429, filed Dec. 30, 2004; and 10/528,503, filed Mar. 27, 2005,now U.S. Pat. No. 7,400,080, each of which is hereby incorporated byreference in its entirety. This application also claims the benefit ofand incorporates by reference in their entirety essential subject matterdisclosed in International Application No. PCT/DK01/00719 filed Oct. 31,2001; German Patent Application No. 100 54 247.6 filed on Nov. 2, 2000;International Application No. PCT/DK02/00862 filed on Dec. 17, 2002;Danish Patent Application No. PA 2001 01933 filed on Dec. 21, 2001;International Application No. PCT/DK2003/000603 filed on Sep. 18, 2003;and Danish Patent Application No. PA 2002 01380 filed on Sep. 20, 2002.

FIELD OF THE INVENTION

The present invention relates to a multilayer composite withelectrically conductive layers and layers of a dielectric materiallocated alternatingly with one another. The multilayer composite can beutilised for conversion between electrical and mechanical energies andcan therefore be used as a sensor, an actuator, a generator, or atransformer—in the following, these four structures will be referred toas transducers.

BACKGROUND OF THE INVENTION

An electrical potential difference between two electrodes located onopposite surfaces of an elastomeric body generates an electric fieldleading to a force of attraction. As a result, the distance between theelectrodes changes and the change leads to compression or tension of theelastomeric material which is thereby deformed. Due to certainsimilarities with a muscle, an elastomer actuator is sometimes referredto as an artificial muscle.

U.S. Pat. No. 6,376,971 discloses a compliant electrode which ispositioned in contact with a polymer in such a way, that when applying apotential difference across the electrodes, the electric field arisingbetween the electrodes contracts the electrodes against each other,thereby deflecting the polymer. Since the electrodes are of asubstantially rigid material, they must be made textured in order tomake them compliant.

The electrodes are described as having an ‘in the plane’ or ‘out of theplane’ compliance. In U.S. Pat. No. 6,376,971 the out of the planecompliant electrodes may be provided by stretching a polymer more thanit will normally be able to stretch during actuation and a layer ofstiff material is deposited on the stretched polymer surface. Forexample, the stiff material may be a polymer that is cured while theelectroactive polymer is stretched. After curing, the electroactivepolymer is relaxed and the structure buckles to provide a texturedsurface. The thickness of the stiff material may be altered to providetexturing on any scale, including submicrometer levels. Alternatively,textured surfaces may be produced by reactive ion etching (RIE). By wayof example, RIE may be performed on a pre-strained polymer comprisingsilicon with an RIE gas comprising 90 percent carbon tetrafluoride and10 percent oxygen to form a surface with wave troughs and crests of 4 to5 micrometers in depth. As another alternative, the electrodes may beadhered to a surface of the polymer. Electrodes adhering to the polymerare preferably compliant and conform to the changing shape of thepolymer. Textured electrodes may provide compliance in more than onedirection. A rough textured electrode may provide compliance inorthogonal planar directions.

Also in U.S. Pat. No. 6,376,971 there is disclosed a planar compliantelectrode being structured and providing one-directional compliance,where metal traces are patterned in parallel lines over a chargedistribution layer, both of which cover an active area of a polymer. Themetal traces and charge distribution layer are applied to oppositesurfaces of the polymer. The charge distribution layer facilitatesdistribution of charge between metal traces and is compliant. As aresult, the structured electrode allows deflection in a compliantdirection perpendicular to the parallel metal traces. In general, thecharge distribution layer has a conductance greater than theelectroactive polymer but less than the metal traces.

The polymer may be pre-strained in one or more directions. Pre-strainmay be achieved by mechanically stretching a polymer in one or moredirections and fixing it to one or more solid members (e.g., rigidplates) while strained. Another technique for maintaining pre-strainincludes the use of one or more stiffeners. The stiffeners are longrigid structures placed on a polymer while it is in a pre-strainedstate, e.g. while it is stretched. The stiffeners maintain thepre-strain along their axis. The stiffeners may be arranged in parallelor according to other configurations in order to achieve directionalcompliance of the transducer.

Compliant electrodes disclosed in U.S. Pat. No. 6,376,971 may compriseconductive grease, such as carbon grease or silver grease, providingcompliance in multiple directions, or the electrodes may comprise carbonfibrils, carbon nanotubes, mixtures of ionically conductive materials orcolloidal suspensions. Colloidal suspensions contain submicrometer sizedparticles, such as graphite, silver and gold, in a liquid vehicle.

The polymer may be a commercially available product such as acommercially available acrylic elastomer film. It may be a film producedby casting, dipping, spin coating or spraying.

Textured electrodes known in the prior art may, alternatively, bepatterned photolithographically. In this case, a photoresist isdeposited on a pre-strained polymer and patterned using a mask. Plasmaetching may remove portions of the electroactive polymer not protectedby the mask in a desired pattern. The mask may be subsequently removedby a suitable wet etch. The active surfaces of the polymer may then becovered with the thin layer of gold deposited by sputtering, forexample.

Producing electroactive polymers, and in particular rolled actuators,using the technique described in U.S. Pat. No. 6,376,971 and U.S. Pat.No. 6,891,317 has the disadvantage that direction of compliance of thecorrugated electrodes is very difficult to control.

Finally, in order to obtain the necessary compliance using the prior arttechnology, it is necessary to use materials having a relatively highelectrical resistance for the electrodes. Since a rolled actuator with alarge number of windings will implicitly have very long electrodes, thetotal electrical resistance for the electrodes will be very high. Theresponse time for an actuator of this kind is given by τ=R·C, where R isthe total electrical resistance of the electrodes and C is thecapacitance of the capacitor. Thus, a high total electrical resistanceresults in a very long response time for the actuator. Thus, in order toobtain an acceptable response time, the number of windings must belimited, and thereby the actuation force is also limited, i.e. responsetime and actuation force must be balanced when the actuator is designed.

SUMMARY OF THE INVENTION

It is an object of a preferred embodiment of the invention to provide adielectric structure which facilitates an increased ratio in theconversion between electrical and mechanical energies as compared tosimilar prior art dielectric structures.

According to a first aspect of the invention the above and other objectsare fulfilled by a multilayer composite comprising at least twocomposites, each composite comprising:

-   -   a film made of a dielectric material and having a front surface        and rear surface, the front surface comprising a surface pattern        of raised and depressed surface portions, and    -   a first electrically conductive layer being deposited onto the        surface pattern, the electrically conductive layer having a        corrugated shape which is formed by the surface pattern of the        film.

Due to the multilayer structure, several advantages are achieved overthe known dielectric structures. The lamination of several compositesprovides an increased conversion between mechanical and electricalenergies not only due to the multiplication of the effect of each layer,but also due to the fact that the multilayer structure itself rendersthe multilayer composite more rigid than a corresponding “one layerdevice” with the identical physical dimensions. In addition, themultilayer structure facilitates application of an electrical field overthinner portions of the structure, thereby requiring much less potentialdifference between electrodes than with a corresponding “one layerdevice” with identical physical dimensions.

The invention of multilayer structures of the corrugated electroactivecomposite also facilitates building of more powerful transducers ascompared to prior art transducers. This being the case as the inventionof lamination of efficient electroactive composites in an unlimitednumber of layers brings an unlimited area of cross section allowing forvery powerful transducers.

The stacking of the mentioned composites also provides a simple andefficient way of arranging electrically conductive layers on bothsurfaces of a film since the electrically conductive layer of one filmbecomes adjacent a film of an adjacent composite. In a multilayercomposite with 10 composites, the 9 composites become electroactive,i.e. they have electrically conductive layers on both surfaces wherebyit can be deformed by application of an electrical potential differencebetween the electrically conductive layers.

The invention of multilayer structures with metallic electrodes havinglow surface resistivity also provides low response times when themultilayer composite is used in a transducer.

The dielectric material could be any material that can sustain anelectric field without conducting an electric current, such as amaterial having a relative permittivity, ∈, which is larger than orequal to 2. It could be a polymer, e.g. an elastomer, such as a siliconeelastomer, such as a weak adhesive silicone or in general a materialwhich has elastomer like characteristics with respect to elasticdeformation. For example, Elastosil RT 625, Elastosil RT 622, ElastosilRT 601 all three from Wacker-Chemie could be used as a dielectricmaterial.

In the case that a dielectric material which is not an elastomer isused, it should be noted that the dielectric material should haveelastomer-like properties, e.g. in terms of elasticity. Thus, thedielectric material should be deformable to such an extent that themultilayer composite is capable of pushing and/or pulling due todeformations of the dielectric material.

The electrically conductive layer may comprise a metal such as silver,gold, or nickel, or an electrically conductive alloy.

The multilayer composite can be made by arranging the composite layersin a stack and by applying an electrical potential difference betweeneach adjacent electrically conductive layer in the stack so that thelayers are biased towards each other while they are simultaneouslyflattened out. Due to the physical or characteristic properties of thefilm, the above method may bond the layers together. As an alternativeor in addition, the layers may be bonded by an adhesive arranged betweeneach layer. The adhesive should preferably be selected not to dampen thecompliance of the multilayer structure. Accordingly, it may be preferredto select the same material for the film and adhesive, or at least toselect an adhesive with a modulus of elasticity being less than themodulus of elasticity of the film.

To enable elongation of the composite in one well defined direction,i.e. to provide compliance, the electrically conductive layers havecorrugated shapes which render the length of the electrically conductivelayers in a lengthwise direction, longer than the length of thecomposites as such in the lengthwise direction. The corrugated shapes ofthe electrically conductive layers thereby facilitate that the compositecan be stretched in the lengthwise direction without having to stretchthe electrically conductive layers in that direction, but merely byevening out the corrugated shape of the electrically conductive layer.According to the invention, the corrugated shape of the electricallyconductive layer is a replica of the surface pattern of the film.

Since the conductive layer is deposited onto the surface pattern of thefilm and is formed by the shape thereof, a very precise shape of thecorrugation of the conductive layer can be defined, and an improvedcompliance towards deformation in a specific direction can be providedby a suitable design of the surface pattern on the film. Accordingly,the composite can facilitate increased actuation forces, or in generalan increased rate of conversion between mechanical and electricalenergies, increased lifetime and improved reaction time when thecomposite is used in a transducer.

In the prior art composites, the pattern of the film and electrode isprovided by stretching the film prior to the application of theelectrode on the surface of the film. When the stretch of the film isreleased, the electrode wrinkles, and since the electrode is bonded tothe film, the surface of the film wrinkles with the electrode. Since theshape of the electrically conductive layer in accordance with thepresent invention is a replica of the shape of the surface pattern ofthe film, it may be provided that the shape of the composite as such isunaffected by the contact and bonding between the electricallyconductive layer and the film. It may further be provided that the shapeis essentially unaffected by elastic moduli of the electricallyconductive layer and film. It may further be unaffected by the thicknessof the electrically conductive layer and film. This provides a largerdegree of freedom with respect to the selection of materials for thefilm and for the electrically conductive layer and thus enables improvedperformance of the composite when used in a transducer.

To restrict deformation of the composite in other directions than thedirection of compliance, the electrically conductive layer may have amodulus of elasticity much higher than a modulus of elasticity of thefilm. Accordingly, the electrically conductive layer resists elongationand thus prevents deformation of the composite in directions in whichthe length of the electrically conductive layer corresponds to thelength of the composite as such.

The composite layers in the multilayer composite should preferably beidentical to ensure a homogeneous deformation of the multilayercomposite throughout all layers, when an electrical field is applied.Furthermore, it may be an advantage to provide the corrugated pattern ofeach layer either in such a manner that wave crests of one layer areadjacent to wave crests of the adjacent layer or in such a manner thatwave crests of one layer are adjacent to troughs of the adjacent layer.

It may be an advantage to arrange the composites with the rear surfacestowards each other. In this way, the multilayer composite becomes lessvulnerable to faults in the film. If the film in one layer has a defectwhich enables short circuiting of electrodes on opposite surfacesthereof, it would be very unlikely that the layer which is arranged withits rear surface against the film in question has a defect at the samelocation. In other words, at least one of the two films provideselectrical separation of the two electrically conductive layers. Thismultilayer structure may be expanded by further composites whereby thecomposites become pair-wise rear to rear and front to front laminated.In this structure two adjacent films in contact with each other reducesthe impact of defects in each film and two adjacent electricallyconductive layers in contact with each other reduces the impact ofdefects in each electrically conductive layer.

As an alternative, the composites may be arranged so that the frontsurface of one composite is towards the rear surface of the adjacentcomposite.

A particular advantage of the multilayer composite is that thecharacteristics of a laminated structure can be used for making themultilayer composite far more rigid than each of the composites innumber of composites sufficient to achieve an area moment of a crosssection for bending of the multilayer composite which is at least 1.5-3times, such as 2 times an average of area moment of inertia of a crosssection of each composite individually.

The composites could be made with a film of a material, e.g. polymericmaterial which has adhesive characteristics. When the composites arestacked, the adhesive characteristics of the film may thus slightly bondthe composites together, e.g. not stronger than what facilitatesde-lamination without damaging each composite. To increase the rigidityof the multilayer composite, the composites could also be adhesivelybonded to each other by use of an additional adhesive. In oneembodiment, they are bonded to a degree where de-lamination isimpossible without destroying the composites. If an additional adhesiveis used, the adhesive may have elastic characteristics similar to thoseof the film. The adhesive could e.g. comprise or consist of the samematerial which constitutes at least a major portion of the film. Inparticular, the adhesive may have a hardness which is at most in theorder of magnitude of that of the film of the composites.

The surface pattern and corrugated shape of composites should preferablybe essentially identical to provide equal expansion characteristics ofeach layer in the multilayer composite, i.e. to enable that each layerexpands equally for an applied electrical field across the films. Insome cases, it may though be desired to establish different expansioncharacteristics of the layers, e.g. by different corrugated shapes ofthe electrically conductive layer of each composite.

The film and the electrically conductive layer may have a relativelyuniform thickness, e.g. with a largest thickness which is less than 110percent of an average thickness of the film, and a smallest thicknesswhich is at least 90 percent of an average thickness of the film.Correspondingly the first electrically conductive layer may have alargest thickness which is less than 110 percent of an average thicknessof the first electrically conductive layer, and a smallest thicknesswhich is at least 90 percent of an average thickness of the firstelectrically conductive layer. In absolute terms, the electricallyconductive layer may have a thickness in the range of 0.01 μm to 0.1 μm,such as in the range of 0.02 μm to 0.09 μm, such as in the range of 0.05μm to 0.07 μm. Thus, the electrically conductive layer is preferablyapplied to the film in a very thin layer. This facilitates goodperformance and facilitates that the electrically conductive layer canfollow the corrugated pattern of the surface of the film.

The electrically conductive layer may have a thickness in the range of0.01-0.1 μm, and the film may have a thickness between 10 μm and 200 μm,such as between 20 μm and 150 μm, such as between 30 μm and 100 μm, suchas between 40 μm and 80 μm. In this context, the thickness of the filmis defined as the shortest distance from a point on one surface of thefilm to an intermediate point located halfway between a crest and atrough on a corrugated surface of the film.

The electrically conductive layer may each have a resistivity which isless than 10⁻⁴ Ω·cm or less than 10⁻⁴ Ω·cm per composite.

By providing an electrically conductive layer having a very lowresistivity the total resistance of the electrically conductive layerwill not become excessive, even if very long electrically conductivelayer are used. Thereby, the response time for conversion betweenmechanical and electrical energy can be maintained at an acceptablelevel while allowing a large surface area of the composite, and therebyobtaining a large actuation force when the composite is used in anactuator. In the prior art, it has not been possible to providecorrugated electrically conductive layer with sufficiently lowelectrical resistance, mainly because it was necessary to select thematerial for the prior art electrically conductive layer with dueconsideration to other properties of the material in order to providethe compliance. By the present invention it is therefore made possibleto provide compliant electrically conductive layers from a material witha very low resistivity, because this allows a large actuation force tobe obtained while an acceptable response time of the transducer ismaintained.

The electrically conductive layer may preferably be made from a metal oran electrically conductive alloy, e.g. from a metal selected from agroup consisting of silver, gold and nickel. Alternatively othersuitable metals or electrically conductive alloys may be chosen. Sincemetals and electrically conductive alloys normally have a very lowresistivity, the advantages mentioned above are obtained by making theelectrically conductive layer from a metal or an electrically conductivealloy.

The dielectric material may have a resistivity which is larger than 10¹⁰Ω·cm. or have a resistivity larger than 10¹⁰ Ω·cm per composite.

Preferably, the resistivity of the dielectric material is much higherthan the resistivity of the electrically conductive layer, preferably atleast 10¹⁰-10¹⁸ times higher.

The corrugated pattern may comprise waves forming crests and troughsextending in one common direction, the waves defining an anisotropiccharacteristic facilitating movement in a direction which isperpendicular to the common direction. According to this embodiment, thecrests and troughs resemble standing waves with essentially parallelwave fronts. However, the waves are not necessarily sinusoidal, butcould have any suitable shape as long as crests and troughs are defined.According to this embodiment a crest (or a trough) will definesubstantially linear contour-lines, i.e. lines along a portion of thecorrugation with equal height relative to the composite in general. Thisat least substantially linear line will be at least substantiallyparallel to similar contour lines formed by other crest and troughs, andthe directions of the at least substantially linear lines defines thecommon direction. The common direction defined in this manner has theconsequence that anisotropy occurs, and that movement of the compositein a direction perpendicular to the common direction is facilitated,i.e. the composite, or at least an electrically conductive layerarranged on the corrugated surface, is compliant in a directionperpendicular to the common direction. In connection with thepotentially unlimited web, the wave crests and troughs may extend e.g.in the lengthwise direction or in the crosswise direction.

Preferably, the compliance of the composite in the compliant directionis at least 50 times larger than its compliance in the common direction,i.e. perpendicularly to the compliant direction.

The waves may have a shape which is periodically repeated. In oneembodiment, this could mean that each of the crests and each of thetroughs are at least substantially identical. Alternatively, theperiodicity may be obtained on a larger scale, i.e. the repeated patternmay be several ‘wavelengths’ long. For instance, the wavelength, theamplitude the shape of the crests/troughs, etc. may be periodicallyrepeated. As an alternative, the shape of the waves may benon-periodically.

Each wave may define a height being a shortest distance between a crestand neighbouring troughs. In this case each wave may define a largestwave having a height of at most 110 percent of an average wave height,and/or each wave may define a smallest wave having a height of at least90 percent of an average wave height. According to this embodiment,variations in the height of the waves are very small, i.e. a veryuniform pattern is obtained.

According to one embodiment, an average wave height of the waves may bebetween ⅓ μm and 20 μm, such as between 1 μm and 15 μm, such as between2 μm and 10 μm, such as between 4 μm and 8 μm.

Alternatively or additionally, the waves may have a wavelength definedas the shortest distance between two crests, and the ratio between anaverage height of the waves and an average wavelength may be between1/30 and 2, such as between 1/20 and 1.5, such as between 1/10 and 1.

The waves may have an average wavelength in the range of 1 μm to 20 μm,such as in the range of 2 μm to 15 μm, such as in the range of 5 μm to10 μm.

A ratio between an average height of the waves and an average thicknessof the film may be between 1/50 and ½, such as between 1/40 and ⅓, suchas between 1/30 and ¼, such as between 1/20 and ⅕.

A ratio between an average thickness of the electrically conductivelayers and an average height of the waves may be between 1/1000 and1/50, such as between 1/800 and 1/100, such as between 1/700 and 1/200.

In a preferred embodiment of the invention the composite is designed byoptimising the parameters defined above in such a manner that dielectricand mechanical properties of the film as well as of the electricallyconductive layer material are taken into consideration, and in such amanner that a composite having desired properties is obtained. Thus, theaverage thickness of the film may be selected with due consideration tothe relative permittivity and breakdown field of the film on the onehand, and electrical potential difference between the electricallyconductive layers on the other hand. Similarly, the height of the crestsmay be optimised with respect to the thickness of the film in order toobtain a relatively uniform electric field distribution across a film ofdielectric material arranged between the electrically conductive layers.Furthermore, electrically conductive layer thickness, averagewavelength, and wave height may be optimised in order to obtain adesired compliance. This will be described further below with referenceto the drawings.

The multilayer composite may comprise a peripheral edge which is coveredwith an electrically isolating layer, e.g. to prevent short-circuitingbetween the electrically conductive layers of the multilayer compositeor in general to protect the edge of the multilayer composite.

In order to benefit mostly from the multilayer composite, e.g. in atransducer comprising a multilayer composite which is curled or windedto form a rolled structure with a larger number of layers or windings,it is preferred to provide the multilayer composite as a very long web.In this context, a web denotes something which is potentially unlimitedin length and which can therefore be provided as a spooled productsimilar to cling-wrap, cling-film or household foil. In general, the webis at least 10 times longer in a lengthwise direction than in aperpendicular crosswise direction, but it may even be 100, 1000 or moretimes longer in the lengthwise direction. The corrugations may extend inany direction relative to the lengthwise and crosswise direction. It is,however, preferred that the corrugations of all the composites making upthe multilayer composite extend in the same direction.

To form a transducer from a multilayer composite, the multilayercomposite must be given a shape and rigidity by which it may interactwith a surrounding system in an intended manner, e.g. as an actuatorcapable of exerting a pressure onto an adjacent object, a sensor capableof sensing a pressure, or as a transformer or generator.

To provide shape and structure, the multilayer composite may e.g. bepre-strained in at least one direction, e.g. by stretching themultilayer composite in a rigid frame—i.e. a frame which is rigidrelative to the multilayer composite. In particular, it may be preferredto stretch the multilayer composite in its direction of compliance, i.e.perpendicular to the direction in which the wave crests and troughsextend. To stretch the multilayer composite, an elastic element could belocated between the rigid frame and the multilayer composite, e.g. aspring or similar elastically deformable element. The multilayercomposite could e.g. be arranged relative to a movable rigid beam orrelative to another multilayer composite so that the composite remainspre-stretched and so that deformation of the multilayer composite can besensed as a change in capacitance measured on two of the electricallyconductive layers, or so that an electrical field can deform the filmsand thus elongate the stretched multilayer composite.

Another way of providing shape and structure is to fold, roll orotherwise stiffen the multilayer composite. This may provide sufficientrigidity for the multilayer composite to exert a force onto an adjacentobject and thereby enable use of the multilayer composite as an actuatorwhich is able to push or pull.

Multilayer composites may be applied as transducers for actuation,sensing and control of components such as valves, flaps, pumps, dosingpumps, etc. Such control components may be applied within industrial,domestic and defence applications such as in hydraulics, industrialautomation and controls, heating and refrigeration, ventilation andair-conditioning, maritime, medical, automotive and off-highwayequipments.

The capacitance of such multilayer composites changes as a function ofthe deflection of the composite. When multilayer composites are used incombination with an electronic control circuit, change in capacitancecan be converted into a control signal for indicating the deflection ofthe transducer. Such control signals can be used as input for improvingclosed loop (feedback) control of transducer position, etc.

Multilayer composites used in combination with control electronics fortransducer positioning can be applied in the above mentionedapplications for accurate responsive control of above mentionedcomponents.

In one specific embodiment, the multilayer composite is made from onesingle continuous film made of a dielectric material. The film isprovided with a front surface with a surface pattern of raised anddepressed surface portions. A first electrically conductive layer e.g.with a corrugated shape is deposited onto a first portion of the surfacepattern, a second electrically conductive layer, e.g. with a corrugatedshape is deposited onto a second portion of the surface pattern. Thefirst and second electrically conductive layers are electricallyisolated from each other e.g. by providing lines or strips in which noelectrically conductive material is deposited onto the film. The film isthen folded, rolled or otherwise formed into a multilayer compositestructure in which the first and second electrically conductive layersare located alternately between layers of the film. In other words, therolling or folding of this “single continuous film” version of themultilayer composite is done both to provide rigidity and to arrange thetwo electrically conductive layers relative to each other. It should benoted that when folding the “single continuous film” care should betaken to fold the film in such a manner that short-circuiting of theelectrodes is prevented, while it is ensured that electrodes of opposingpolarity are positioned opposite each other with a layer of dielectricfilm there between.

To form a transducer, electrical connectivity between the electricallyconductive layers and an electronic circuit or power supply isnecessary. The above-mentioned “single continuous film” may have onlytwo such electrically conductive layers since it is one single layerwhich is folded or rolled. Other types of multilayer composites,however, comprise a plurality of individual electrically conductivelayers—one for each composite in the multilayer composite. In this case,connection between every second electrically conductive layer and oneconnector of the electronic circuit or power supply, and connectionbetween each intermediate electrically conductive layer and anotherconnector of the electronic circuit or power supply, may be establishedby providing a multilayer composite wherein the electrically conductivelayers are shifted relative to each other for every second layer. As anexample, the film of the composites form dielectric layers having firstand second surfaces towards the electrically conductive layers. Theelectrically conductive layers may define:

-   -   an active portion of the actuator wherein electrode portions of        the electrically conductive layers cover both surfaces of the        dielectric layers;    -   a first passive portion and a second passive portion in which        portions only one surface of the dielectric layers is covered by        one of the conductive layers;        The first passive portion is defined by a contact portion of the        electrically conductive layer on the first surface, and the        second passive portion is defined by a contact portion of the        electrically conductive layer on the second surface.

An electrical connector can thereafter be attached to each contactportion to connect the contact portions to the power supply orelectronic circuit. As an example, conductive rods, bolts, nails, screwsor rivets may penetrate all layers of the multilayer composite througheach contact portion and thus be used for electrically connecting themultilayer composite to a power supply or to an electronic circuit.

In a second aspect, the invention provides a method of providing amultilayer composite of layers of a dielectric film and an electricallyconductive layer. The method comprises:

-   -   providing a first composite comprising a film of a dielectric        material with a front surface and an opposite rear surface, the        front surface comprising a surface pattern of raised and        depressed surface portions,    -   depositing an electrically conductive layer onto the surface        pattern,    -   providing a second composite comprising a film with a front        surface and an opposite rear surface, the front surface        comprising a surface pattern of raised and depressed surface        portions,    -   depositing an electrically conductive layer covering at least a        portion of the surface pattern,    -   arranging the first composite on the second composite, and    -   fixating the position of the first composite relative to the        second composite.

The last step of fixating the position could be done by use of adhesivecharacteristics of the film itself or by use of an additional adhesiveprovided between the composites. Alternatively, the layers could befixed to one common fixation means, e.g. by attaching all layers to onecommon frame or flange or similar component which can keep the layerstogether.

Preferably the multilayer composite is made as a spooledproduct—potentially in unlimited length. For this purpose, the steps ofproviding the composites and arranging them in a stack could beperformed in a continuous process, wherein a part of the multilayercomposite is made while another part of that multilayer composite isfinished and being rolled up.

The composites could e.g. be provided by providing a shape definingelement having a surface pattern of raised and depressed surfaceportions and by providing a liquid polymer composition onto the surfacepattern to form a first film having a surface with a replicated patternof raised and depressed surface portions. This could be done e.g. in acoating, moulding, painting, spraying or in any similar process in whicha liquid polymer can be brought onto a surface with a surface pattern.To provide identical layers in the multilayer composite, the same, oridentical shape defining elements could be used throughout the process.

The electrically conductive layer could be deposited onto the replicatedsurface pattern by use of a physical vapour deposition process, in asputtering process, or in an electron beam process. The electricallyconductive layer is deposited onto the film in a thickness of 0.01-0.1μm, and the thickness could be controlled by quartz crystal microbalance.

Quartz crystal micro balance is a thickness measurement technique thatis commonly used in physical vapour deposition. It allows forcontrolling the thickness of the deposited coating, e.g. a metal coatingor similar, with accuracies in the sub-nanometer range.

Prior to the application of the electrically conductive layer, the filmmay be treated with plasma since this may improve adhesion of theelectrically conductive layer to the film. The plasma treatment could beconducted with a glow discharge which is known to generate mild plasma,and argon plasma is preferred.

Plasma cleaning is a critical step in the metallization process ofelastomer films. It enhances adhesion of the deposited material.However, not any plasma is appropriate for treating the elastomer film,and the plasma should therefore be selected carefully. As mentionedabove, argon plasma is preferred. Plasma treatments are known to formthin and very stiff silicate “glassy” layers at the elastomer interface.When an electrically conductive layer is subsequently applied, theresult is corrugated electrodes with limited compliance and compositeswhich cannot be stretched very much because of the risk of cracking thestiff electrodes. We have chosen the argon plasma treatment which is notreactive because argon is a noble gas. However, residues of oxygen andother reactive gases in the vacuum deposition chamber combined with theargon plasma, are responsible for a little reactivity. We optimise thepressure of argon in the vacuum chamber and the parameters of the mildglow discharge, as well as the duration of the treatment in such a waythat the deposited metal coating adheres very well to the elastomerfilm. The resulting corrugated electrode is very compliant and thecomposite can be stretched as much as allowed without damaging theelectrode according to the design rules described in previousparagraphs.

An adhesion promoter could be applied to the film after the plasmatreatment. The adhesion promoter may comprise a layer of chromium ortitanium, and the adhesion promoter could be applied to the film in aphysical vapour deposition process.

In an alternative process, a liquid polymer composition is provideddirectly onto the first film or onto the electrically conductive layerto provide a second film by moulding or coating directly onto the firstfilm. The replicated pattern could in this case be provided bysimultaneous embossing with a stamping tool into the not yet cured orpartly cured polymer. When the polymer is cured, at least a portion ofits pattern could be coated with a second electrically conductive layer.The process could be repeated to provide a multilayer composite with alarge number of layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference tothe accompanying drawings in which:

FIGS. 1 a and 1 b illustrate continuous rolls of spooled compositesaccording to embodiments of the invention,

FIG. 1 c is a perspective view of a portion of a composite according toan embodiment of the invention,

FIGS. 2 a-2 f are cross sectional views of a portion of compositesaccording to embodiments of the invention,

FIG. 2 g is an enlarged section of FIG. 2 a/2 b/2 c/2 d/2 e/2 f,

FIGS. 3 a and 3 b show an electroactive composite being exposed to zeroelectrical potential difference and being exposed to a high electricalpotential difference,

FIGS. 4 a-4 c illustrate the effect of exposing the electroactivecomposite of FIG. 3 a to a high electrical potential difference as shownin FIG. 3 b,

FIGS. 5 a and 5 b illustrate an example of lamination of compositesaccording to an embodiment of the invention, thereby forming anelectroactive multilayer composite,

FIGS. 5 c and 5 d illustrate an electroactive multilayer composite beingexposed to zero electrical potential difference and being exposed to ahigh electrical potential difference,

FIGS. 6 a and 6 b illustrate another example of lamination of compositesaccording to an embodiment of the invention, thereby forming anelectroactive multilayer composite,

FIGS. 6 c and 6 d illustrate another electroactive multilayer compositebeing exposed to zero electrical potential difference and being exposedto a high electrical potential difference,

FIGS. 7-9 illustrate examples of lamination principles of compositesaccording to embodiments of the invention,

FIGS. 10 a and 10 b illustrate examples of rolled electroactivecomposites,

FIG. 11 a illustrates an example of a portion of a composite accordingto an embodiment of the invention, the composite being particularlysuitable for a composite having a rolled structure,

FIG. 11 b illustrates an example of a portion of a composite accordingto an embodiment of the invention, the composite being particularlysuitable for a composite having a folded structure,

FIGS. 12 a-12 c illustrate a process of making the composite of FIG. 11and some of the tools needed for the production,

FIG. 13 a illustrates the composite of FIG. 11 a formed as a rolledcomposite,

FIG. 13 b illustrates the composite of FIG. 11 b formed as a foldedcomposite,

FIGS. 14 a and 14 b illustrate lamination of the composite shown FIG. 11by folding of the composite,

FIGS. 15 a-15 c are perspective views of direct axially actuatingtransducers according to embodiments of the invention,

FIGS. 16 a-16 c are graphs illustrating force as a function of stroke ina direct actuating transducer according to an embodiment of theinvention,

FIGS. 17 a and 17 b are perspective views of direct radially actuatingtransducers according to embodiments of the invention,

FIG. 18 a illustrates lamination of a composite to form a flat tubularstructure,

FIG. 18 b illustrate the flat tubular structure of FIG. 18 a beingpre-strained,

FIGS. 19 a-19 c are perspective views of an actuating transducer havinga flat structure,

FIGS. 20 a-20 e illustrate actuating transducers provided with apreload,

FIGS. 21 a and 21 b illustrate two actuating transducers having a flattubular structure, the transducers being provided with mechanicalconnection,

FIG. 22 illustrates the principle of space-shifted laminated layers ofcomposites,

FIG. 23 illustrates laminated electroactive multilayer compositesprovided with electrical contact portions and electrical connectors,

FIGS. 24 and 25 illustrate two examples of electroactive multilayercomposites provided with electrical contact portions,

FIGS. 26-29 illustrate examples of transducers provided with electricalcontact portions,

FIG. 30 illustrates different electrical connectors,

FIGS. 31-35 illustrate electroactive composites provided with contactelectrodes,

FIGS. 36 a-36 c is a process diagram describing a manufacturing processof a transducer according to an embodiment of the invention, and

FIG. 37 illustrates a partially cut view of a multilayer compositehaving an electrically isolating layer according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b illustrate continuous rolls of spooled composites 1according to embodiments of the invention, and FIG. 1 c is a perspectiveview of a portion of a composite 1. The proportions of the composite aredistorted in order to illustrate different elements of the composite 1.The composite 1 comprises a film 2 made of a dielectric material havinga surface 3 provided with a pattern of raised and depressed surfaceportions, thereby forming a designed corrugated profile of the surface3. An electrically conductive layer 4 has been applied to the surface 3,the electrically conductive material being deposited so that theelectrically conductive layer is formed according to the pattern ofraised and depressed surface portions. In terms of everyday physicalthings, the film 2 resembles in some aspects household wrapping film. Ithas a similar thickness and is comparably pliable and soft. However, itis more elastic than such a film, and has a marked mechanical anisotropyas will be explained in the following.

The dielectric material may be an elastomer or another material havingsimilar characteristics.

Due to the pattern of raised and depressed surface portions, theelectrically conductive layer 4 may even out as the film 2 expands, andrecover its original shape as the film 2 contracts along the directiondefined by arrows 5 without causing damage to the electricallyconductive layer 4, this direction thereby defining a direction ofcompliance. Accordingly, the composite 1 is adapted to form part of acompliant structure capable of withstanding large strains.

As described above, the corrugated surface profile is directly impressedor moulded into the dielectric film 2 before the electrically conductivelayer is deposited. The corrugation allows the manufacturing of acompliant composite using electrode materials of high elastic modulii,e.g. metal electrode. This can be obtained without having to applypre-stretch or pre-strain to the dielectric film 2 while applying theelectrically conductive layer 4, and the corrugated profile of thefinished composite 1 does not depend on strain in the dielectric film 2,nor on the elasticity or other characteristics of the electricallyconductive layer 4. Accordingly, the corrugation profile is replicatedover substantially the entire surface 3 of the dielectric film 2 in aconsistent manner, and it is possible to control this replication.Furthermore, this approach provides the possibility of using standardreplication and reel-to-reel coating, thereby making the processsuitable for large-scale production. For instance, the electricallyconductive layer 4 may be applied to the surface 3 of the dielectricfilm 2 using standard commercial physical vapour deposition (PVD)techniques. An advantage of this approach is that the anisotropy isdetermined by design, and that the actual anisotropy is obtained as aconsequence of characteristics of the corrugated profile which isprovided on the surface 3 of the dielectric film 2 and the electricallyconductive layer 4 which follows the corrugated profile.

The composite 1 shown in FIG. 1 c is designed to have a compliance inthe range of the compliance of the dielectric film 2 in the directiondefined by arrows 5, and a stiffness in the range of the stiffness ofthe electrically conductive layer 4 in a direction defined by arrows 6.In FIG. 1 a, the compliance direction is along the length of thecomposite 1, whereas the compliance direction of FIG. 1 b is across thecomposite 1. This is indicated by the thin lines across the composite 1in FIG. 1 a and along the composite 1 in FIG. 1 b, which thin linesrepresents the pattern of raised and depressed surface portions formingthe corrugated profile. The composite 1 may be produced in very longlengths, so called “endless” composites which may be stored as spools asshown in FIGS. 1 a and 1 b. Such semi finished goods may be used for theproduction of transducers and the like, e.g. actuators.

FIGS. 2 a-2 f illustrate a portion of a sectional view of composites 1according to embodiments of the invention, with hatchings omitted forthe sake of clarity. As indicated by the symmetry line 10 at the bottomof each portion, each portion only shows half a composite 1.Furthermore, an electrically conductive layer 4 may be deposited on thelower surface of the dielectric film 2, which lower surface may alsodefine a corrugated surface, thereby forming an electroactive composite,i.e. at least two electrically conductive layers being separated by adielectric film. Furthermore, each portion only shows a small portionlengthwise of each composite. For illustration purposes the proportionsof FIGS. 2 a-2 g are out of order. FIG. 2 g illustrates an enlargedsection of FIG. 2 a/2 b/2 c/2 d/2 e/2 f. The composite 1 shown in FIGS.2 a-2 g could, e.g., be the composite 1 of FIG. 1 a. Thus, the composite1 comprises a dielectric film 2 made of a dielectric material having asurface 3 provided with a pattern of raised and depressed surfaceportions, thereby forming a corrugated profile of the surface 3. Thesurface 3 is provided with an electrically conductive layer (shown inFIG. 2 g) forming a directionally compliant composite as describedabove. As shown in FIGS. 2 a-2 f, the pattern of raised and depressedsurface portions may be designed having various shapes.

The corrugated profile may be represented by a series of well definedand periodical sinusoidal-like three dimensional microstructures.Alternatively, the corrugated profile may have a triangular or a squareprofile. The mechanical compliance factor, Q, of the corrugatedelectrode is determined by the scaling ratio between the depth d of thecorrugation and the thickness h (see FIG. 2 g) of the electricallyconductive layer 4, and by the scaling ratio between the depth d of thecorrugation and its period P. The most dominating factor is the scalingratio between the height d of the corrugation and the thickness h of theelectrically conductive layer 4. The larger the compliance factor, themore compliant the structure is. It has been found by the inventors ofthe present invention, that if perfect compliance is assumed, for ascaling ratio between the depth d of the corrugation and its period P, asinus profile could theoretically elongate approximately 32%, atriangular profile approximately 28% and a square profile approximately80% compared to the original length. However, in reality this will notbe the case since the square profile comprises vertical and horizontalbeams, which will result in different compliances, because the verticalbeams will bend and thereby generate a very compliant movement in thedisplacement direction, while the horizontal beams will be much stiffer,since they extent in the displacement direction. It is therefore oftendesirable to choose the sinus profile.

In the composite 1 shown in FIGS. 2 a-2 f, the corrugated patternimpressed or moulded into the dielectric film 2 can be represented by aseries of well defined and periodical sinusoidal-like three dimensionalmicrostructures. The corrugation profile is formed at the upper surface3 of the film 2 as shown in FIG. 2 a-2 f. As indicated by the symmetryline 10, a second corrugation profile is formed at the lower surface(not shown) of the film. In FIGS. 2 a-2 f, the section runs along thedirection of compliance. Perpendicularly to the direction of complianceparallel straight lines represent tops and bottoms of the raised anddepressed surface portions, i.e. wave crests or troughs of thesinusoidal-like microstructure. This appears more clearly from FIGS. 1 aand 1 c. Along these parallel straight lines, the compliancy is verylow, i.e. for all practical purposes the composite 1 is not compliant inthis direction. In other words, this design represents a one dimensionalcorrugation which, upon application of the electrically conductivelayers, transforms the dielectric film 2 into an electrocative composite1 with anisotropic compliance, wherein the film is free to contract orelongate, while a perpendicularly arranged cross-plane direction is‘frozen’ due to the built-in boundary conditions given by the mechanicalresistance of the electrically conductive layers 4.

In FIGS. 2 a-2 g, d denotes an average or representative corrugationdepth, i.e. an average or representative distance between a raisedportion and a neighbouring depressed portion of the pattern. H denotesan average thickness of the dielectric film 2, and h denotes an averagethickness of the electrically conductive layer 4. In a preferredembodiment, the average thickness H of the dielectric film 2 is in therange of 10 μm-100 μm. FIGS. 2 a-2 c show composites 1 having differentcorrugation depth d, whereas the corrugation period P is substantiallyidentical for the three composites shown. Comparing the composites 1 ofFIGS. 2 d and 2 e, the corrugation depth d is substantially identical,whereas the corrugation period P of the composite 1 in FIG. 2 e islarger than the corrugation period P of the composite 1 shown in FIG. 2d. Compared hereto, the composite 1 of FIG. 2 f has a smallercorrugation depth d and a larger corrugation period P.

The properties of the dielectric films 2 with anisotropic corrugatedcompliant metallic electrodes in the form of electrically conductivelayers 4 as described in accordance with the present invention areoptimised by design according to design rules developed by theinventors. These design rules take into consideration the dielectric andmechanical properties of the dielectric material and of the material ofthe electrically conductive layer.

The relative permittivity and breakdown field of the dielectric materialon the one hand and electrical potential difference between electrodeson the other hand are the design parameters that determine the range ofthe average thickness, H of the dielectric film 2. The characteristicproperties of the dielectric material are typically supplied bydielectric material manufacturers like Wacker-Chemie and Dow Corning.

Corrugation depth, d, is optimised with respect to the dielectric filmthickness, H, in order to obtain a relatively uniform electric fielddistribution across the dielectric film situated between the electrodes.Such optimisation step is done using finite element simulations. A highd/H ratio corresponds to a non uniform electric field distribution and alow d/H ratio corresponds to a relatively uniform electric fielddistribution.

Anisotropy and compliance properties are the combined result of theshape and topology given to the surface of the dielectric film, e.g. anelastomer film, by a moulding process on one hand and the electricallyconductive layer that takes up the corrugation shape on the other hand.Electrode layer thickness, h, and corrugation period, P, are optimisedwith respect to the corrugation depth, d, in order to obtain adielectric film with metallic electrodes that is compliant in one ‘inthe plane’ direction and almost not compliant in the transverse ‘in theplane’ direction. A film that is very compliant in one direction is afilm that can be stretched or elongated very much in this direction byapplying a relatively low level of forces in this direction without therisk of damaging the electrodes, and a film that will have very limitedelongation in the transverse direction when a force is applied in thistransverse direction. In order to optimise electrode compliance, the d/Pand h/d ratios have to be optimized. High d/P ratios result in verycompliant electrodes and low d/P ratios result in less compliantelectrodes. High h/d ratios result in less compliant electrodes and lowh/d ratios result in very compliant electrodes. The degree of anisotropyof the dielectric film with corrugated electrodes is determined by thecompliance ratio between the direction in which the composite iscompliant and the transverse direction in which the composite is almostnot compliant. High compliance ratios result in very anisotropicstructures and low ratios result in isotropic-like structures.

Once the ranges for the design parameters (H, d, h and P) are specifiedaccording to the above description, it is possible to predict theperformance of the dielectric film with metallic electrodes in the formof electrically conductive layers in terms of how compliant and whatmaximum elongation in the compliant direction it can undergo and whatthe actuation forces will be. Stiffness in the transverse direction canbe predicted as well. A refinement process for these parameters can bedone if necessary.

It should be noted that for a given actuation force, actuatorsmanufactured in accordance with the present invention, i.e. made from adielectric material with electrodes deposited thereon, has a much lowerweight, i.e. at least a factor five smaller, than conventionalactuators, such as magnetic actuators, capable of providing a comparableactuation force. This is very important for applications where actuatorvolume and weight are of relevance.

Once all design parameters are optimised, a mould is designed accordingto the exact specifications for the corrugation topology.

Based on finite element electrostatic simulations, the inventors of thepresent invention have found that the ratio d/H should be in the rangeof 1/30-½. For example, having a ratio of 1/5 and a corrugation depth ofapproximately 4 μm, the thickness of the dielectric film 2 will beapproximately 20 μm. Furthermore, the ratio between the corrugationdepth d and the period P of the corrugations, d/P, and the ratio betweenthe thickness h of the electrically conductive layer and the corrugationdepth d, h/d, are important ratios directly affecting the compliance ofthe electrode. In preferred embodiments, the ratio d/P is in the rangeof 1/50-2, whereas the ratio h/d is in the range of 1/1000-1/50.

Another issue to take into consideration when defining the averagethickness H of the dielectric film 2 is the so-called breakdown electricfield related to dielectric materials. When an electrically conductivelayer 4 is deposited on each surface of the dielectric film 2 therebyforming an electroactive composite, there is a maximum value for thevoltage, V between these electrically conductive layers, for a givenmaterial thickness, H, i.e. a distance corresponding to the thickness,H, of the dielectric film 2, in order not to exceed the breakdownelectric field, V/H, of the material. When the dielectric film 2presents large variations in thickness across a surface area 3, then,for a given voltage between the electrically conductive layers, electricfield and thickness variations will be of the same order of magnitude.As a consequence, parts of the dielectric film 2 having a higher localelectric field will elongate more than those with a smaller localelectric field. Furthermore, in situations where a transducer in whichthe composite 1 is operated close to a breakdown field, such variationsmay be damaging to the transducer, because parts of the dielectric film2 will be subjected to electric fields which are larger than thebreakdown field. Accordingly, it is very important to reduce the averagethickness variations to the greatest possible extent when processing thedielectric film 2. For processing reasons a 10% average thicknessvariation is considered acceptable. When processing transducers withcorrugated electrodes by design, i.e. in accordance with the presentinvention, these values can be controlled in a relatively accuratemanner.

FIGS. 3 a and 3 b illustrate an electroactive composite 1 comprising twoelectrically conductive layers 4 separated by a dielectric film 2 beingexposed to zero electrical potential difference (FIG. 3 a) and beingexposed to a high electrical potential difference (FIG. 3 b). Asillustrated in FIG. 3 b the dielectric film 2 is expanded, while theelectrically conductive layers 4 are evened out, when exposed to anelectrical potential difference. This is shown in detail in FIGS. 4 a-4c which illustrate portions of a section of the electroactive composite1 at different steps in time, with hatchings omitted for the sake ofclarity. A line of symmetry 10 is indicated at the bottom of eachfigure, illustrating that the composite 1 is an electroactive compositehaving an electrically conductive layer 4 deposited on each surface.FIG. 4 a illustrate the electroactive composite 1 being exposed to zeroelectrical potential difference, the corrugation depth being thedesigned depth d and the corrugation period being the designed period P.In FIG. 4 b it is illustrated that the dielectric film 2 is expanded inthe compliance direction resulting in a reduced thickness H′ of thefilm. Furthermore, the electrically conductive layer 4 is evened outresulting in a smaller corrugation depth d′ and a larger corrugationperiod P′. FIG. 4 c illustrate the electroactive composite 1 at a latertime step, the thickness H″ of the film 2 being even more reduced, thecorrugation depth d″ being even smaller and the corrugation period P″being larger.

It should be noted that capacitors produced in accordance with thepresent invention exhibit a ‘self-healing’ mechanism. A self-healingmechanism is characteristic of capacitors with very thin electrodes. Itoccurs when the dielectric material of the capacitor presents defectssuch as inclusions, pinholes, etc. For such a capacitor with a giventhickness, when the applied potential difference between electrodesapproaches the so-called breakdown voltage defined above, the averageelectric field approaches the critical breakdown field. However, inregions with defects, it will indeed exceed this critical breakdownfield, and a cascading effect due to accelerated and colliding chargesacross dielectric film thickness at the positions of the defects occurs,thereby inducing a high in-rush transient current across the dielectricmaterial. This results in a local transient over-heating withcharacteristic times in the microseconds range or much below, which isenough to “deplete/evaporate” the material of the very thin oppositeelectrodes at the positions of the defects and their close vicinity.This results in areas around defects where there is no more electrodematerial. Moreover the dimension of the areas with depleted electrodematerial increases with the local field. However, the capacitor as suchis not damaged and continues to operate. Thus, the reference to‘self-healing’. As long as the depleted areas represent in total a verynegligible fraction of the entire area of the capacitor, this will havevery little consequence on the performance of the capacitor.Self-healing does not take place if the capacitor is made with thickelectrodes, because the level of local over-heating is not sufficient todeplete the thick electrode material at the defects. In that case, whenthe critical breakdown is reached, consequent and instant damage of thecapacitor occurs. In practice, the inventors of the present inventionhave made metallic electrodes with thickness up to 0.2 μm and alwaysobserved self-healing, even when operating the capacitor abovebreakdown. This does not cause any substantial damage to the capacitor,and the capacitor therefore continues to operate.

FIGS. 5-9 illustrate examples of lamination of composites 1 therebycreating multilayer composites. As shown in FIGS. 5 a and 6 a, anelectroactive multilayer composite 15, 16 comprises at least twocomposites 1, each composite 1 comprising a dielectric film 2 having afront surface 20 and a rear surface 21, the rear surface 21 beingopposite to the front surface 20. The front surface 20 comprises asurface pattern 3 of raised and depressed portions and a firstelectrically conductive layer (not shown) covering at least a portion ofthe surface portion 3. FIGS. 5 a and 6 a only show a portion of amultilayer composite 15 and 16, which portions having proportions out oforder for illustration purposes.

FIGS. 5 a and 5 b show an electroactive multilayer composite 15 havingthe first composite 1 arranged with its front surface 20 facing the rearsurface 21 of the adjacent composite 1, in the following referred to ingeneral as a Front-to-Back multilayer composite 15. In this type oflamination process, the electrically conductive layer of the firstcomposite 1 is in direct contact with the rear surface of the secondcomposite 1. The composites 1 are laminated either by the use of anelastomer of the same type as used for producing the dielectric film 2or alternatively, the two composites 1 are stacked without use of anadhesive. For some purposes it is preferred that the multilayercomposite is made of stacked composites without the use of an adhesive.In these cases, the wave troughs are simply filled with air.

Due to the pattern of raised and depressed surface portions 3, theelectrically conductive layer of each of the composites may even out asthe film expands, and recover its original shape as the film contractsalong the direction defined by arrows 5 (see FIG. 5 b) without causingdamage to the electrically conductive layers, this direction therebydefining a direction of compliance. Thus, the multilayer composite 15shown in FIG. 5 b is designed to be very compliant in the directiondefined by arrows 5 and designed to be very stiff in the transversedirection defined by arrows 6.

FIGS. 5 c and 5 d illustrate the electroactive multilayer composite 15being exposed to zero electrical potential difference and being exposedto a high electrical potential difference. As can be seen from FIG. 5 dthe dielectric film is expanded, while the electrically conductivelayers are evened out, when exposed to an electrical potentialdifference. It can further be seen that the depth of the wave troughs(the corrugation depth d) is reduced when the multilayer composite isexposed to an electrical potential difference. The composites can bebonded by applying a high electrical potential difference to the stackedcomposites, whereby the film of one composite and the electricallyconductive layer of an adjacent composite adhere to each other withoutthe use of an additional adhesive. Thus, they may be brought intointimate contact by electrostatic forces. Alternatively, they may adhereto each other by pressing them together, e.g. by the use of rollers, dueto the characteristics of the dielectric film which may be slightlytacky when made of an elastomer.

As an alternative hereto, FIGS. 6 a and 6 b show an electroactivemultilayer composite 16 having the first composite 1 arranged with itsrear surface 21 facing the rear surface 21 of the adjacent composite 1,in the following referred to in general as a Back-to-Back multilayercomposite 16. The composites 1 are adhesively bonded either by the useof an elastomer adhesive with characteristics similar to the dielectricfilm 2 of the composites 1. Alternatively, the two composites 1 arestacked without use of an adhesive.

In the electroactive multilayer composite 16 illustrated in FIG. 6 a,the corrugated surfaces 3 can be coated with the electrically conductivelayer before or after laminating the composites 1. The Back-to-Backmultilayer composite 16 has the advantage that the impact of defects inthe dielectric film 2, pin-holes in the electrically conductive layeretc. may become less critical if the adjacent layer does not havesimilar errors in close vicinity.

If the individual composites 1 are made in identical production steps,there may be an increased risk that identical errors exist on the samelocation of each composite 1. To reduce the impact of such errors, itmay be an advantage to shift the location of one composite 1 relative toan adjacent composite 1, or to rotate the composites 1 relative to eachother. Additionally, as seen in FIG. 37, the multilayer composite 16 maycomprise a peripheral edge 129 which is covered with an electricallyisolating layer 130, e.g. to prevent short-circuiting between theelectrically conductive layers of the multilayer composite or in generalto protect the edge of the multilayer composite. FIG. 37 only shows aportion of a multilayer composite 16, which has proportions out of orderfor illustration purposes.

The lamination process represents a critical step in the productionprocess. Thus, precise lamination machines equipped with tension controlare to be used.

Similar to the multilayer composite 15 the multilayer composite 16 shownin FIG. 6 b is designed to be very compliant in the direction defined byarrows 5 and designed to be very stiff in the transverse directiondefined by arrows 6.

FIGS. 6 c and 6 d illustrate the electroactive multilayer composite 16being exposed to zero electrical potential difference and being exposedto a high electrical potential difference. As can be seen from FIG. 6 dthe dielectric film is expanded, while the electrically conductivelayers are evened out, when exposed to an electrical potentialdifference.

FIG. 7 a illustrates that an electroactive multilayer composite 15 ofthe kind illustrated in FIG. 5 a may further contain an endless numberof composites 1 depending on the specific need. The multilayer compositein FIG. 5 a contains one dielectric film 2 out of two dielectric films 2which is inactive, i.e. only one of the two dielectric films 2 islocated between two electrically conductive layers (not shown). FIG. 7 aillustrates that a larger number of composites decreases the impact ofthe inactive layers on the electroactive multilayer composite 15 assuch, since all but the lowermost composite 15 are located betweenelectrodes.

FIG. 7 b illustrates an alternative way of forming an electroactivemultilayer structure 15 containing an endless number of composites 1.The composites 1 have been laminated by means of adhesive layers 22arranged between the composites 1 in such a manner that the composites 1are not in direct contact with each other. The material of the adhesivelayers 22 has properties similar to those of the dielectric material ofthe composites 1, in terms of ability to stretch. This is in order toallow the adhesive layers 22 to stretch along with the dielectricmaterial when the multilayer structure 15 is working. Thus, the adhesivelayers 22 may advantageously be made from an elastomer, or from amaterial with elastomer-like properties.

In FIG. 8, two electroactive multilayer composite 16 of the kind alsoshown in FIG. 6 a, i.e. Back-to-Back composites, are stacked on top ofeach other. In this electroactive multilayer composite, the electricallyconductive layers are pair-wise in contact with each other. Twodielectric films 2 are located between two of such sets of twoelectrically conductive layers. The laminate offers a reduced impact ofproduction defects in the individual layers. Furthermore, it isillustrated that a third or even further electroactive multilayercomposite(s) 16 may be added to this multilayer composite.

FIG. 9 illustrates a stack of multilayer composites 16 similar to thestack shown in FIG. 8. However, in the situation illustrated in FIG. 9,the Back-to-Back multilayer composites 16 are stacked pair-wise, and thepair-wise stacked multilayer composites 16 are then stacked together. Inthe stack illustrated in FIG. 9 it is ensured that the electricallyconductive layers of adjacent pair-wise stacks facing each other has thesame polarity. Accordingly, such a stack can be rolled without riskingshort-circuiting of the electrodes, and the stack is therefore suitablefor being rolled, e.g. to form a tubular transducer.

FIG. 10 a illustrates a Front-to-Back electroactive multilayer composite15 as shown in FIG. 5 a being rolled. Since the composite 1 may beproduced in very long lengths, so called “endless” composites, themultilayer composite 15 may also be produced in very long lengths,thereby allowing for the producing for rolled multilayer compositescomprising numerous windings.

FIG. 10 b illustrates rolling of a multilayer composite 15 around rods23. The rods 23 are positioned at an end of the multilayer composite 15,and the composite 15 is then rolled around the rods 23 as indicated.Thereby the multilayer composite 15 obtains a rolled tubular shape.

FIGS. 11 a and 11 b show a portion of a composite 24 which is suitablefor forming a rolled or otherwise laminated transducer. The composite 24comprises a film 2 made of a dielectric material having a surfaceprovided with a pattern of raised and depressed surface portions,thereby forming a designed corrugated profile of the surface, i.e. thefilm 2 is similar to the film 2 of the composite 1 of FIG. 1 c. In thiscase the film 2 is provided with an electrically conductive layercomprising negative electrode portions 25 and positive electrodeportions 26 arranged in an interleaved pattern, i.e. the negativeelectrode portions 25 and the positive electrode portions 26 appearalternating with a gap in between. In the gap an electrically conductivelayer is not deposited on the dielectric film. The arrow 27 indicatesthat the composite 24 may be a very long, an “endless”, composite asshown in FIG. 13 a, and as a folded composite as shown in FIG. 13 b.

FIGS. 12 a-12 c illustrate one possible method of making the composite24 of FIG. 11. FIG. 12 a illustrates the film 2 being a very long filmon two rolls 30. The electrically conductive layer (not shown) isdeposited on the film 2 using a non-continuous vapour deposition roll toroll method. The arrows 31 indicate the process direction. Theelectrically conductive layer is deposited through a shadow mask 32 inorder to provide gaps in between the electrode portions 25, 26. When theelectrically conductive layer is deposited on an area of the film 2, thefilm 2 is rolled in the direction of the arrows 31 and stopped. Ashutter (not shown) is opened and the electrically conductive layer isdeposited on the next area of the film 2, this area being adjacent tothe previous area, and ensuring a continuous transition contact betweenelectrodes with the same polarity. The shutter is closed when therequired thickness of the electrically conductive layer is achieved. Theelectrode deposition principle where electrodes are deposited through ashadow mask is, for practical reasons, more appropriate for productionof electrodes with constant width and gap. As an alternative, the gapmay be made by means of laser ablation. In fact, it is preferred to makethe gap by means of laser ablation, since when using such a technique itis very easy to provide a variable distance between each gap and thus avariable width of each portion of the electrically conductive layer.This will be explained in further detail below.

FIG. 13 a illustrates the composite 24 a of FIG. 11 a and FIGS. 12 a-12b formed as a rolled composite 35. D and R denote diameter and radius ofa roll 36 onto which the composite 24 is rolled. The solid lines denotepositive electrodes while the dotted lines denote negative electrodes.It should be noted that for the sake of clarity, the rolled composite isshown by means of concentric circles. However, it should be understoodthat in reality the rolled composite forms a spiral pattern. The width,w, of the electrode portions 25 and 26 and the width of the gap betweenthese electrode portions are determined based on the cross section ofthe roll 36 as follows: 2π(R)=w+gap, where the gap is very small ascompared to w. Furthermore, it is preferred that the thickness t of thecomposite 24 a is smaller than the gap. Otherwise, the efficiency of thetransducer which is formed by this roll process becomes low. When awinding n is made by rolling the composite 24 a, the gap is tangentiallyshifted by a film thickness order, 2πt·n with respect to the previouswinding. Thus if the gap shift exceeds the gap width, electrodes withsame polarity will tend to overlap, and this renders the correspondingportions of the capacitor inactive. This method is preferred forbuilding actuators with limited number of windings and operating in apre-strained configuration or flat tubular actuator configurations whereelectrode portions and gaps are deposited in the portions of dielectricweb that correspond to flat portions of the flat tubular actuator. Analternative method where laser ablation is used to design the electrodeswith variable width but constant gap width is more appropriate for therolled tubular actuator. In this case, the width of the gap and depletedregions is determined by the traveling laser spot size, and the width ofa given electrode associated to a given winding of the growingcircumference of the actuator is such that width and gap match thewinding circumference.

Similarly, FIG. 13 b illustrates the composite 24 b of FIG. 11 b as afolded composite 37. It is clear from FIG. 13 b that the composite 24 bis folded carefully in such a manner that it is ensured that electrodes25, 26 of opposite polarity do not come into direct contact.

FIGS. 14 a and 14 b illustrate lamination of the composite shown FIG. 11by folding of the composite 24. Alternatively, the composite could be ofthe kind shown in FIGS. 1 a and 2. The composite 1, 24 is manufacturedin a long structure, thereby defining a length and a width of thecomposite 1, 24, and has a surface 3 with a pattern of raised anddepressed surface portions. The pattern defines waves of crests andtroughs, extending in a common direction, and the common direction isarranged substantially along the width of the long structure.Accordingly, the composite 1, 24 is compliant in a directionperpendicular to the common direction, i.e. along the length of the longstructure.

The composite 1, 24 of FIG. 14 a is laminated by folding the longstructure along the length, i.e. in such a manner that the width of theresulting electroactive multilayer composite 40 is identical to thewidth of the composite 1, 24. Due to the orientation of the compliantdirection of the composite 1, 24 the electroactive multilayer composite40 will be compliant in a direction indicated by arrows 41.

FIG. 14 b illustrates lamination of a composite 1, 24 according toanother embodiment of the invention. This is very similar to theembodiment shown in FIG. 14 a. However, in this case the commondirection is arranged substantially along the length of the longstructure, and the composite 1, 24 is therefore compliant in a directionalong the width of the long structure, as the composite of FIG. 1 b.Accordingly, the resulting electroactive laminate 42 will be compliantin a direction indicated by arrows 43.

Thus, the laminated composite shown in FIG. 14 a is compliant along thelength of the laminated composite. This means that the structure of FIG.14 a can be made to be of any length, and thus of any desired strokelength. Similarly, the laminated composite of FIG. 14 b is compliantalong the width of the laminated composite. This means that thestructure of FIG. 14 b can be made to be of any width. Thus, it ispossible to design a transducer with any appropriate dimensions inaccordance with geometrical requirements of the intended application.

FIGS. 15 a-15 c are perspective views of direct actuating transducers 50according to embodiments of the invention. The direct actuatingtransducer 50 of FIGS. 15 a-15 c have been manufactured by rolling amultilayer composite, e.g. of the kind shown in FIG. 1 a or in FIG. 5.The transducer 50 a of FIG. 15 a is solid, whereas the transducer 50 bof FIG. 15 b is hollow. The transducers 50 may have any elongated form,e.g. substantially cylindrical with a cross section which issubstantially circular, elliptical or curve formed as illustrated inFIG. 15 c.

In FIGS. 15 a-15 c the composite, which has been rolled to form thecolumnar shaped transducers 50, has a direction of compliance which isparallel to the directions indicated by arrows 51. Accordingly, whenelectrical energy is applied to the electrodes of the direct actuatingtransducers 50, the transducers 50 will elongate axially in thedirection of the arrows 51. It has now been found that if thetransducers 50 are properly made and dimensioned in accordance withcertain aspects of the invention, they are able to exert significantforce against an axial load which tends to resist the axial elongation.

As indicated earlier in this specification, the electroactive compositeof the present invention is quite supple and pliable, resemblingordinary household cling film or polyethylene shopping bag sheetmaterial in pliability. The composite differs from those materials byits higher elasticity and its mechanical anisotropy, as previouslyexplained, being very stretchy in one direction and much less stretchyin the perpendicular direction.

The inventors now have realised that despite of the suppleness,pliability and elasticity of the composite, a roll formed by winding upa sufficient length of the composite will be quite stiff. If the roll isproperly wound with respect to the mechanical anisotropy of the film, itwill have axial compliance brought about by the mechanical anisotropy,and yet it can be quite resistant to buckling under axial load.

Accordingly, a composite of corrugated anisotropic dielectric filmlayers with electrically conductive electrode layers can be rolled intoa tubular shape with a number of windings sufficient to make theresulting structure of the tubular element sufficiently stiff to avoidbuckling. In the present context, the term ‘buckling’ means a situationwhere an elongated structure deforms by bending due to an applied axialload. It has been found that no additional component such as anystiffening rod or spring inside the elongated structure is necessary toobtain sufficient stiffness to avoid buckling under technically usefullevels of axial load. The required stiffness is obtained merely bywinding up a sufficient number of windings of the composite material.

The rolled structures illustrated in FIGS. 15 a-15 c are designed towithstand a specified maximum level of load at which the stiffness issufficient to avoid buckling. This specified maximum level may, e.g., bea certain level of force at a certain level of elongation, or it may bea maximum level of actuation force, a blocking force, or a higher levelof force occurring when the transducer is compressed to a shorter lengthagainst the direction of the arrows 51.

Design parameters for the direct actuating transducer as described inthe present application are optimised according to design rulesdeveloped by the inventors. These design rules allow for determining theoptimum dimensions of a rolled actuator (transducer) based on theactuator performance specifications.

The mechanical and electrostatic properties of an electroactivecomposite are used as a basis to estimate actuator force per unit areaand stroke. Rolled actuators as described in accordance with the presentinvention are made by rolling/spooling very thin electro-activecomposites, e.g. as shown in FIGS. 1 a and 1 b, having a thickness inthe micrometers range. A typical actuator of this type can be made ofthousands of windings and can contain as many as 100 windings permillimeter of actuator wall thickness.

When activated, direct/push actuators convert electrical energy intomechanical energy. Part of this energy is stored in the form ofpotential energy in the actuator material and is available again for usewhen the actuator is discharged. The remaining part of mechanical energyis effectively available for actuation. Complete conversion of thisremaining part of the mechanical energy into actuation energy is onlypossible if the actuator structure is not mechanically unstable, likethe well-known buckling mode of failure due to axial compression. Thiscan be achieved by properly dimensioning the cross-sectional area of theactuator in relation to actuator length. Mathematically this correspondsto Euler's theory of column stability; in accordance with the invention,this theory also applies to an actuator column formed by rolling up asufficient number of windings of electroactive multilayer composite.

The optimisation process starts by defining the level of force requiredfor a given application. Then based on the actuator force per unit area,it is possible to estimate the necessary cross sectional area to reachthat level of force.

For a cylindrical structure, the critical axial load or force F_(c) fora given ratio between length and radius of the cylinder is given by:

${F_{C} = \frac{c \cdot \pi^{2} \cdot E \cdot A}{\left( {L/R} \right)^{2}}},$where c is a boundary condition dependent constant,

-   E is the modulus of elasticity,-   A is the cross sectional area of the cylinder,-   L is the length of the cylinder, and-   R is the radius of the cylinder.

Consider now an electro-active polymer transducer of cylindrical shapewhich is actuated by applying a voltage, V, to its electrodes. In theunloaded state, the transducer will simply elongate. If restrained by anaxial load, the transducer will exert a force upon the load whichincreases with the voltage, V. The maximum force, F_(max), which thetransducer can be actuated to depends on the construction of thetransducer.

For a given length L and cross section A, this means that the voltageneeds to be controlled in such a manner that forces higher thanF_(max)<F_(C) are not allowed. For a given cross section, this meansthat the length of the cylinder must be smaller than a critical length,L_(C), i.e. L<L_(C), with L_(C) defined as follows.

For a transducer 50 with a given cross section and a chosen maximumforce level, the maximum force level being related to the maximumvoltage level, the critical length, L_(C), can be derived from theformula:

${L_{C} \leq \sqrt{\frac{c \cdot r^{2} \cdot \pi^{2} \cdot E}{F_{\max}/A}}},$and the design criteria is L<L_(C).

For a selected voltage level a transducer 50 with a given cross sectionis able to actuate with a given maximum force, the so-called blockingforce, F_(bl), at 0% elongation. In this situation the design criterionis:

$L_{C} = {\sqrt{\frac{c \cdot r^{2} \cdot \pi^{2} \cdot E}{F_{bl}/A}}.}$

Applying these design criteria for a transducer 50 made of an elastomerwith E=1 MPa, F_(bl)/A=20 N/cm² and c=2, the design rule forF_(max)=F_(bl) will be L_(bl)=10·r, i.e. the so-called slendernessratio, λ, must fulfil the following condition in order to obtain anon-buckling structure at the load being equal to the blocking force:λ≦L/r=10.

For alternatively chosen lower levels for the actuating force for thesame transducer 50, i.e. for a cylindrically symmetric transducer 50with the same radius, r, the design criteria for length L can be derivedfrom the following formula:L≦L _(bl)·√{square root over (F _(bl) /F)}.

This may, e.g., mean that if the actuation level at 10% elongation is¼·F_(bl), then the length, L, of that transducer at 10% elongation is:L≦L _(bl) ·√{square root over (1/¼)}= L _(bl)·2.

The theory of Euler can be applied to designing a transducer 50 with aspecific need for transducer stroke and a chosen percentage ofelongation of the dielectric film. Since there is no limitation toincrease in cross sectional area, A, of the cylindrical symmetrictransducer 50 a and 50 b due to an increased number of windings, andbecause the design rules derived from the theory of Euler are fulfilled,it is possible to simply provide the necessary number of windings toobtain a required level of actuation force. Accordingly, the technologydescribed above makes it possible to build dielectric transducers havingnon-buckling characteristics at a given force level and a given strokefor direct actuation.

When designing a direct acting capacitive transducer, it is necessary todimension its mechanical structure against buckling. This is donetypically by increasing the area moment of inertia of its cross section,known as I. As an example, a piece of paper with a given thickness (h),width (w) and length (L) will bend when a little force is applied to thepaper in a direction parallel to its length. However, by rolling it inthe width direction, a much larger force will be necessary to make itbuckle. Rolled-to-flat bending stiffness ratio is then given by

$\frac{3}{2} \cdot {\left( {1 + \left( \frac{w}{h/\pi} \right)^{2}} \right).}$An example of such is to take w=40 mm and h=1 mm, then the ratio isabout 245.

Stabilisation of the actuator against any mechanical instabilityrequires dimensioning its cross section by increasing its area moment ofinertia of the cross section I. Low values of I result in less stablestructures and high values of I result in very stable structures againstbuckling. The design parameter for dimensioning the structure is theradius of gyration r_(g) which relates cross section A and area momentI. Low values of r_(g) result in less stable actuator structures andhigh values of r_(g) result in very stable actuator structures. Afterhaving defined optimum ranges for both area A and radius of gyrationr_(g), it is possible to define the optimum range for the rolledactuator wall thickness, t, with respect to r_(g) in the form oft/r_(g). Area A, radius r_(g) and wall thickness t are the designparameters for dimensioning the actuator cross-section for maximumstability. Low values of t/r_(g) result in very stable actuatorstructures and high values of t/r_(g) result in less stable actuatorstructures.

Once the ranges of the cross section parameters have been determined, itis necessary to estimate the maximum length L of the actuator for whichbuckling by axial compression does not occur for the required level offorce. Slenderness ratio defined as the length L to radius of gyrationr_(g) ratio is the commonly used parameter in relation with Euler'stheory. Low values of L/r_(g) result in very stable actuator structuresand high values of L/r_(g) result in less stable actuator structuresagainst buckling.

Once all design parameters for the optimum working direct actuator havebeen determined, it is possible to estimate the total number of windingsthat are necessary to build the actuator based on the actuator wallthickness t and the number of windings per millimeter n for a givenelectro-active composite with a specific thickness in the micrometerrange.

In a preferred embodiment, the ratio between the number n of windingsand the wall thickness t of the transducer, n/t, should be in the rangeof 10 windings/mm-50 windings/mm. Furthermore, the slenderness ratio,being the ratio between the length L of the transducer and the gyrationradius r_(g) of the transducer should be less than 20. The gyrationradius r_(g) is defined as r_(g)=√{square root over (I/A)}, where I isthe area moment of a cross section and A is the cross sectional area ofthe transducer.

Thus, by carefully designing transducers in accordance with the presentinvention, it is possible to obtain large actuation forces, even thougha very soft dielectric material is used. Actuation forces may even reachlevels comparable to conventional transducers made from hardermaterials, e.g. magnetic transducers. This is a great advantage.

FIG. 16 a is a graph illustrating force as a function of stroke in adirect actuating transducer according to an embodiment of the invention.When voltage is applied to the anisotropic compliant electricallyconductive layers of the transducer, electric field induced compressionacross film thickness is converted into elongation/stroke along thecompliant direction of the transducer. The corresponding stress isreferred to as Maxwell stress, P, and the corresponding actuation forceis referred to as electrostatic force F_(electrostatic). Uponelongation, the dielectric material exerts a counterforce F_(elastomer)which increases with transducer stroke as shown in FIG. 16 a.

Consequently, the effective force available for direct actuation F_(act)is a result of the two described forces, andF_(act)=F_(electrostatic)−F_(elastomer), as shown in FIG. 16 b. Thecharacteristic curve representing force versus stroke of the directactuating transducer is typical for force transducers, where actuationforce decreases as a function of increasing stroke, until a maximumvalue of the stroke is reached corresponding to “zero” actuation forceas depicted in FIG. 16 b.

FIG. 16 c illustrates the range of calculated direct actuation forces asa function of transducer stroke for different outer diameters of adirect acting capacitive transducer, a rolled transducer. Largeactuation forces in the range of hundreds to thousands of Newtons can begenerated. Blocking forces are typically 4 orders of magnitude largerthan nominal actuations forces defined at 10% transducer stroke. Adirect acting capacitive transducer made of a 40 micrometer thickdielectric material with elastic modulus in the range of 0.5-1 MPa willgenerate a force per unit area in the range of 0.1-0.2 N/mm², for atypical actuation voltage of 3000 volts. When considering largetransducer cross sections, this corresponds to large actuation forces asshown in FIG. 16 c.

FIGS. 17 a and 17 b are perspective views of direct actuatingtransducers 52 according to alternative embodiments of the invention.The transducers 52 of FIGS. 17 a and 17 b have a direction of compliancealong the tangent of the cylinder. Accordingly, the elongation of thetransducers 52 takes place on a perimeter of the tubular structure,illustrated by the arrows 53, i.e. the transducer 52 is caused to expandand contract in a radial direction.

FIG. 18 a illustrates lamination of a composite 1 to form a flat tubularstructure 60. The composite 1 may advantageously be of the kind shown inFIGS. 1 a and 2. The transducer 60 is a laminate of a sufficiently highnumber of adhesively bonded composites to ensure a rigidity of thetransducer, which rigidity is sufficient to enable that the transducercan work as an actuator without being pre-strained. The transducer 60 ismanufactured by winding a continuous composite, e.g. of the kind shownin FIGS. 1 a and 2, in a very flat tubular structure. Using this designthe limitations regarding number of layers described above areeliminated. Thereby, the transducer 60 can be made as powerful asnecessary, similarly to what is described above with reference to FIGS.15 a-15 c.

The flat tubular structure of the transducer 60 shown in FIG. 18 a isobtained by rolling the composite 1 around two spaced apart rods 61 toform a coiled pattern of composite 1. Due to the orientation of thecompliant direction of the composite 1, the flat tubular structure 60will be compliant in a direction indicated by arrows 62. FIG. 18 billustrate the transducer of FIG. 18 a being pre-strained by two springs63.

FIGS. 19 a-19 c are perspective views of transducers 70 having a flatstructure. The transducer 70 is a multilayer composite of a sufficientlyhigh number of adhesively bonded composites to ensure a rigidity of thetransducer, which rigidity is sufficient to enable that the transducercan work as an actuator without being pre-strained. The transducer 70 ismanufactured by laminating a continuous composite, e.g. of the kindshown in FIGS. 1 a and 2, in a flat structure. Using this design thelimitations regarding number of layers described above are eliminated.Thereby, the transducer 70 can be made as powerful as necessary,similarly to what is described above with reference to FIGS. 15 a-15 c.The transducer 70 a is a multilayer composite of a sufficiently highnumber of adhesively bonded composites to ensure a rigidity of thetransducer, which rigidity is sufficient to enable that the transducercan work as an actuator without being pre-strained. The transducer 70 bis dimensioned by stacking a number of transducers 70 a. As analternative hereto, the transducer 70 c may be pre-strained by a spring71 or by other elastically deformable elements.

The transducer 70 a and 70 b is provided with fixation flanges 72 inorder to attach the transducer in an application, e.g. in order for thetransducer to work as an actuator. The arrows 73 indicate the directionof compliance.

FIGS. 20 a-20 e illustrate actuating transducers 80 provided with apreload. FIG. 20 a is a perspective view of a flat transducer 80provided with fixation flanges 81. The flat transducer 80 of FIG. 20 ais pre-strained by a spring 82. Accordingly, the flat transducer 80 hasa direction of actuation indicated by arrows 83. FIG. 20 b illustrates asimilar flat transducer 80 in which the spring is replaced by a similarsecond flat transducer 80. FIG. 20 c illustrates half of a transducer,the transducer being similar to the transducer of FIG. 20 b anddimensioned by the use of a number of identical transducers (only halfof them are shown). FIGS. 20 d and 20 e illustrate two alternativetransducers 84 and 85 each comprising a number of flat transducers 80being pre-strained by adjacent transducers similar to the transducer ofFIG. 18 b. The transducers 84 and 85 actuate cross directional, in FIG.20 d in a carpet-like structure and in FIG. 20 e in a wall-likestructure.

It should be noted that the transducers of FIGS. 18-20 only requirepre-strain along one direction, i.e. in the direction of compliance.Thus, a pre-strain in a direction transverse to the direction ofcompliance, which is necessary in prior art transducers, is not requiredin transducers according to the present invention.

FIG. 21 a illustrates two pre-strained transducers 90 having a flattubular structure, the transducers 90 actuating in the longitudinaldirection and thereby rotating an actuating shaft 91.

FIG. 21 b illustrates two mechanically pre-strained flat transducers 92,93 provided with mechanical connection 94, which is supported by aguiding element for sliding purposes. The transducers 92, 93 are shownin three situations. In the first situation neither of the transducers92, 93 are active. However, they are both mechanically pre-strained. Inthe second situation, transducer 93 is active. Since the transducer 92is inactive, the transducer 93 causes transducer 92 to relax, therebyreleasing some of the mechanical pre-strain of transducer 92. In thethird situation transducer 92 is active while transducer 93 is inactive.Transducer 92 thereby causes transducer 93 to relax, thereby releasingsome of the mechanical pre-strain of transducer 93. Thus, thetransducers 92, 93 in combination with the mechanical connection 94 forma double-acting transducer in which one of the transducers causes theother transducer to relax and release mechanical pre-strain.

FIG. 22 illustrates an electroactive composite comprising a dielectricfilm 2 with a first surface 100 and a second surface 101 being oppositeto the first surface 100. Both surfaces of the dielectric film 2 arepartly covered with an electrically conductive layer. Due to the shapeand location of the electrically conductive layers, an active portion Aexists, in which electrode portions 102, 103 of the electricallyconductive layers cover both surfaces 100, 101 of the dielectric film 2.The electrically conductive layers further define a first passiveportion B in which only the second surface 101 of the dielectric film 2is covered by a contact portion 104 of one of the conductive layers anda second passive portion C in which only the first surface 100 of thedielectric film 2 is covered by a contact portion 105 of the otherconductive layer. As it appears, the electroactive composite can beelectrically connected to a power supply or connected to control meansfor controlling actuation of the composite by bonding conductors to thecontact portions 104, 105. Even if the illustrated composite islaminated, rolled, or folded to form a transducer with a large number oflayers, the electrode portions 102, 103 may easily be connected to apower supply e.g. by penetrating the layers in each contact portion 104,105 with an electrically conductive wire or rod and by connecting thewire or rod to the power supply. The ratio between the thickness of thedielectric film 2 and the thickness of the electrically conductivelayers is merely for illustration purposes. The process illustrated inFIG. 22 may be referred to as ‘off-set’, since the contact portions 104,105 are provided by applying the electrode portions 102, 103 on thesurfaces 100, 101 of the dielectric film 2 ‘off-set’ relatively to eachother.

FIGS. 23 a-23 c illustrate three different ways of space shifting twocomposites 1 of a multilayer composite forming a transducer where eachcomposite 1 comprises an electrically conductive layer on a dielectricfilm. The illustrated composites 1 have a compliance direction in whichthey expand or contract when the transducer is activated. In FIG. 23 a,the contact portions are space shifted along the compliance direction,in FIG. 23 b, the contact portions are space shifted perpendicular tothe compliance direction, and in FIG. 23 c, the contact portions arespace shifted both in the compliance direction and in a direction beingperpendicular to the compliance direction. In any of the configurations,it is desired to keep the region where the physical contact is madebetween the multilayer composite and the connecting wire, rod or similarconductor away from any source of stress or moving parts. FIG. 23 dillustrates the multilayer composite in a side view.

Thus, FIGS. 22 and 23 a-23 c illustrate two different principles forproviding contact portions 104, 105, i.e. the ‘off-set’ principle inFIG. 22 and the ‘space shifting’ principle in FIGS. 23 a-23 c. Theseprinciples may be combined with various lamination processes, and aprinciple which is appropriate for the intended application mayaccordingly be chosen.

FIG. 24 illustrates that contact portions 104, 105 form part ofelectrically conductive layers and form extension islands on one side ofthe electrode portions 102 and 103. The islands of two adjacentcomposites in a multilayer composite are located differently so that thecontact portions 104, 105 of adjacent composites are distant from eachother.

FIG. 25 illustrates two composites each provided with an electricallyconductive layer. When the composites are joined in a multilayerstructure, they are offset relative to each other so that a portion ofthe electrically conductive layer on each composite forms a contactportion 104 being distant from the corresponding contact portion 105 onthe other composite.

FIGS. 26 and 27 illustrate tubular transducers 50 as shown also in FIGS.15 a and 15 b. The tubular transducers are connected to a power supplyat the indicated contact portions 104, 105.

FIG. 28 illustrates a transducer 110 with a flat tubular structure. Thetransducer comprises contact portions 104, 105 on an inner surface. Thecontact portions may be connected to a power supply e.g. via one of theelongated rods 111 with electrically conductive contact portions. Therod 111 is shown in an enlarged view in FIG. 29 in which it can be seenthat the rod 111 comprises two contact portions 112, 113 which come intocontact with the contact portions 104, 105 of the flat tubular structurewhen the rod 111 is inserted into the tubular structure. The rods 111could form part of a device on which the transducer operates. Bothspace-shifted and off-set electrode principles can be applied incontacting the above described transducer structure.

FIG. 30 shows three different kinds of connectors, i.e. a soft connector120, a metal coated plastic connector 121, and a metal or metal coatedgrid strip connector 122. The soft connector 120 comprises an elastomerfilm 123 coated with a layer of electrically conductive material 124.Similarly, the metal coated plastic connector 121 comprises a plasticportion 125 coated with a metal layer 126.

FIGS. 31-35 illustrate composites 1 provided with electrical contacts.Since the composite 1 of the present invention is very soft, it is achallenge to join the composite 1 to a somewhat stiffer normalelectrical connector, such as a wire, a strip, a grid, etc.

FIG. 31 shows a soft connector 120 connected to a composite 1 comprisinga dielectric film 2 with a corrugated surface 3 provided with a layer ofelectrically conductive material 4. The electrically conductive parts124, 4 of the soft connector 120 and the composite 1, respectively, havebeen joined via a layer of electrically conductive adhesive 127, therebyelectrically connecting the composite 1 and the soft connector 120.

FIG. 32 shows two composites 1 having been joined as described above,i.e. via a layer of electrically conductive adhesive 127, and thecomposite 1 positioned on top is used as main electrode to a powersupply.

FIG. 33 shows a metal or metal coated wire or strip 128 connected to acomposite 1. The metal or metal coated wire or strip 128 is adapted tobe connected to a main power supply. Similarly to what is describedabove, the metal or metal coated wire or strip 128 is joined to theelectrically conductive layer 4 of the composite 1 by means of anelectrically conductive adhesive 127. However, in this case theelectrically conductive adhesive 127 is arranged in such a manner thatit surrounds a periphery of the metal or metal coated wire or strip 128,thereby providing a very efficient electrical contact between the metalor metal coated wire or strip 128 and the electrically conductive layer4 of the composite 1.

FIG. 34 shows a metal or metal coated grid strip connector 122 connectedto a composite 1 via an electrically conductive adhesive 127. Asdescribed above with reference to FIG. 33, the electrically conductiveadhesive 127 is arranged in such a manner that a part of the metal ormetal coated grid strip connector 122 is completely surrounded, therebyproviding a very good electrical contact.

FIG. 35 shows a metal coated plastic connector 121 connected to acomposite 1 via a layer of electrically conductive adhesive 127. Asdescribed above with reference to FIGS. 31 and 32, the layer ofelectrically conductive adhesive 127 is arranged between the metal layer126 of the metal coated plastic connector 121 and the electricallyconductive layer 4 of the composite 1, thereby providing electricalcontact there between.

FIG. 36 a illustrates the process of manufacturing a tool or mould forthe process of making the composite, e.g. a composite 1 as illustratedin FIG. 1. FIG. 36 b illustrates the process of manufacturing thecomposite by use of the tool, and FIG. 36 c illustrates the process ofmaking a transducer from the composite.

Thus, we start the process by making a master mould having the desiredcorrugation profile. We may fabricate the mould by laser interferencelithography on photoresist coated glass, or by standard photolithographyon silicon wafers.

For the standard photolithography on silicon wafers, the exposure maskis relatively simple and may preferably exhibit equally spaced andparallel lines, e.g. having a width of 5 μm and a spacing of 5 μm.Standard silicon micromachining recipes are then used to etch thesilicon in order to form so-called V-grooves, i.e. grooves having across sectional shape resembling a ‘V’. A series of oxidation andhydrofluoric acid etching steps are then performed to transform theV-grooved structures into quasi-sinusoidal corrugations, if this is thedesired shape.

We can fabricate master moulds of a relatively large size, such as up to32 cm×32 cm, by means of laser interference lithography. In laserinterference lithography two laser beams, each with an expanded spotsize and with uniform energy distribution across the beam cross section,are caused to interfere onto a photoresist coated glass substrate. Sucha process does not require any exposure mask, and relies on theinterference phenomenon known in the field of optics. The result ofexposure, development and, finally, hard-baking, is a direct sinuswaveform profile written onto the photoresist, where profile period andamplitude are determined by the laser beam wavelength, the incidenceangles of the laser beams onto the photoresist, and the thickness of thephotoresist.

In the next step of the process as illustrated in FIG. 36, we usestandard stress-free electroplating processes to fabricate a sufficientnumber of nickel copies or moulds necessary in order to obtainreplication of corrugated microstructures onto plastic rolls. Thesenickel replicas also called shims have a thickness in the 100 micrometerrange. These shims are mechanically attached in a serial configurationto form a “belt” having a total length which is precisely set to matchthe circumference of the embossing drum. Use of thin shims facilitatesbending them without building too much stress and subsequently rollingthe “belt” around the drum circumference. Each shim is placed withrespect to its neighbours in such a way that corrugation lines areadjusted with micrometer accuracy for minimising any angularmisalignment between lines of neighbouring shims. Then the corrugatedmicrostructures of the embossing drums, resulting from the nickelmoulds, are accurately replicated onto plastic rolls. We may do so bymeans of roll-to-roll micro embossing (UV or heat curing). Roll-to-rollembossing allows for the production of rolls of micro-embossed plasticmaterial having lengths in the range of hundreds of meters. We use themicro-embossed plastic rolls as carrier web, e.g. in the form of a beltor a mould, for the production of dielectric films having single-surfaceor double-surface corrugations, e.g. elastomer films having lengths inthe range of hundreds of meters.

We fabricate corrugated elastomer films or sheets of limited size bywell known spin coating. It is a discontinuous process, and the maximumsize of the film or sheet is determined by the size of the mould.Alternative types of production processes are the kinds developed forthe polymer industry, such as adhesive tapes, painting, etc., normallyreferred to as ‘roll-to-roll coating’ or ‘web coating’. These productionprocesses are large scale, large volume, and continuous processes.

In a subsequent step, we fabricate elastomer films using themicro-embossed plastic roll, e.g. using a roll-to-roll, reverse roll,gravure, slot die, bead or any other suitable kind of coating technique.As a result an elastomer coated plastic film is obtained. To this endreverse roll and gravure roll coating techniques are considered the mostpromising among other known techniques because they offer coatings whichare uniform and have a relatively well defined thickness. We select thesurface properties of the embossed plastic roll or mould and of theembossing resin in a manner which allows for wetting by the elastomermaterial. We carry out the production process of the elastomer film in aclean room environment in order to fabricate pinhole-free elastomerfilms of high quality.

We expose non-cured elastomer film formed onto the mould as describedabove to heat, ultraviolet light or any other source capable ofinitiating cross-linking, in order to cause the elastomer film to cure.The chosen source will depend on the type of elastomer material used, inparticular on the curing mechanism of the used material.

Then we release the cured film from the mould in a delamination process.To this end appropriate release tooling is used. We may preferablychoose mould material and elastomer material to facilitate the releasingprocess. Very weak adhesion of cured elastomer to the substrate mould ispreferred. If very good adhesion occurs, the release process can failand damage the film. A single-sided corrugated elastomer film roll isthe product of this delamination process.

In the next step we deposit the metal electrode onto the corrugatedsurface of the elastomer film by means of vacuum web metallization.Accordingly, a metal coating, e.g. a coating of silver, nickel, gold,etc., is applied to the corrugated surface. Thus, a composite is formed.

The challenge in the large scale manufacturing of elastomer film havinglengths in the range of kilometers is not in the production of flatfilms, but rather in the production of single-sided or double-sidedcorrugated film with precise and very well defined micro structures.Another challenge is in handling these very soft materials usingcontrolled tension forces which are several orders of magnitude smallerthan the control tension forces normally occurring in the polymerindustry. Metallization of a corrugated elastomer film surface withreliable coating layers, when the thickness of a coating layer is only1/100 of the depth of the corrugated pattern, is yet another challengingissue of the production process.

Next, we laminate the coated elastomer films, the composites, therebyforming a multilayer composite, as described above. Then we roll themultilayer composite to form the final rolled transducer structure. Therolled transducer undergoes finishing and cutting, and electricalconnections are applied.

Finally, we may integrate the finished transducer into a final productalong with control electronics, and the transducer is ready for use.

While the present invention has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisinvention may be made without departing from the spirit and scope of thepresent invention.

1. A multilayer composite comprising at least two composites, each composite comprising: a film made of a dielectric material and having a front surface and rear surface, the front surface comprising a surface pattern of raised and depressed surface portions, and a first electrically conductive layer being deposited onto the surface pattern, the electrically conductive layer having a corrugated shape which is formed by the surface pattern of the film, wherein the at least two composites are arranged with front surface facing front surface, and wherein at least one further composite is arranged with its rear surface facing a rear surface of one of the at least two composites.
 2. The multilayer composite according to claim 1, wherein the dielectric material is a polymer.
 3. The multilayer composite according to claim 1, wherein the multilayer composite is made from a number of composites sufficient to achieve an area moment of a cross section for bending of the multilayer composite which is at least 2 times an average of an area moment of inertia of each composite individually.
 4. The multilayer composite according to claim 1, wherein the composites are adhesively bonded to each other.
 5. The multilayer composite according to claim 1, wherein the surface pattern of the film of each composite is substantially identical.
 6. The multilayer composite according to claim 1, wherein the surface pattern of the film of each composite comprises waves forming crests and troughs extending in one common direction, the waves defining a compliance of the electrically conductive layers to deform in a direction perpendicular to the common direction and thereby an anisotropic characteristics of the multilayer composite.
 7. The multilayer composite according to claim 6, wherein the waves have a shape which is periodically repeated.
 8. The multilayer composite according to claim 6, wherein each wave defines a height being a shortest distance between a crest and neighbouring troughs, each wave having a height that deviates at most 10 percent from an average wave height.
 9. The multilayer composite according to claim 6, wherein the film has an average thickness being between 10 and 200 μm.
 10. The composite according to claim 6, wherein a ratio between an average height of the waves and an average thickness of the film is between 1/50and ½.
 11. The composite according to claim 6, wherein the waves have a wavelength defined as the shortest distance between two crests, and wherein a ratio between an average height of the waves and an average wavelength is between 1/30and
 2. 12. The composite according to claim 6, wherein a ratio between an average thickness of the first electrically conductive layer and an average height of the waves is between 1/1000 and 1/50.
 13. The multilayer composite according to claim 6, wherein the composites are arranged relative to each other to provide a shortest possible distance between crests of one layer and crests of another layer.
 14. The multilayer composite according to claim 6, wherein the composites are arranged relative to each other to provide a longest possible distance between crests of one layer and crests of another layer.
 15. A multilayer composite comprising at least two composites, each composite comprising: a film made of a dielectric material and having a front surface and rear surface, the front surface comprising a surface pattern of raised and depressed surface portions, and a first electrically conductive layer being deposited onto the surface pattern, the electrically conductive layer having a corrugated shape which is formed by the surface pattern of the film, wherein the at least two composites are arranged with front surface facing front surface, wherein the surface pattern of the film of each composite comprises waves forming crests and troughs extending in one common direction, the waves defining a compliance of the electrically conductive layers to deform in a direction perpendicular to the common direction and thereby an anisotropic characteristics of the multilayer composite, and wherein the multilayer composite is pre-strained in a direction perpendicular to the direction of the crests and troughs.
 16. A multilayer composite comprising at least two composites, each composite comprising: a film made of a dielectric material and having a front surface and rear surface, the front surface comprising a surface pattern of raised and depressed surface portions, and a first electrically conductive layer being deposited onto the surface pattern, the electrically conductive layer having a corrugated shape which is formed by the surface pattern of the film, wherein the at least two composites are arranged with front surface facing front surface, and wherein any number of such multilayer composites are arranged with rear surface facing rear surface.
 17. A multilayer composite comprising at least one composite, the at least one composite comprising: a film made of a dielectric material and having a front surface and rear surface, the front surface comprising a surface pattern of raised and depressed surface portions; a first electrically conductive layer being deposited onto the surface pattern, the first electrically conductive layer having a corrugated shape which is formed by the surface pattern of the film; and a second electrically conductive layer being deposited onto the surface pattern, the second electrically conductive layer having a corrugated shape which is formed by the surface pattern of the film; wherein the first and second electrically conductive layers are arranged on the surface pattern in an interleaved pattern with a gap therebetween, the gap electrically isolating the first electrically conductive layer from the second electrically conductive layer.
 18. The multilayer composite according to claim 17, wherein the composite is rolled to form the multilayer composite, the rolled multilayer composite having layers that alternate between the first electrically conductive layer and the second electrically conductive layer.
 19. A method of making a multilayer composite with a film made of dielectric material and having a front surface and rear surface, the front surface comprising a surface pattern of raised and depressed surface portions, the method comprising: depositing a first electrically conductive layer on the front surface of a film; and depositing a second electrically conductive layer on the front surface of the film; wherein the first and second electrically conductive layers are deposited on the surface pattern in an interleaved pattern with a gap therebetween, the gap electrically isolating the first electrically conductive layer from the second electrically conductive layer.
 20. The method according to claim 19, wherein the first and second electrically conductive layers are deposited simultaneously onto the surface pattern through a shadow mask defining the interleaved pattern and the gap.
 21. The method according to claim 19, wherein the gap is formed by laser ablation. 